A high speed silicon Mach-Zehnder modulator is proposed based on interleaved PN junctions. This doping profile enabled both high modulation efficiency of V(π)L(π) = 1.5~2.0 V·cm and low doping-induced loss of ~10 dB/cm by applying a relatively low doping concentration of 2 × 10(17) cm(-3). High speed operation up to 40 Gbit/s with 7.01 dB extinction ratio was experimentally demonstrated with a short phase shifter of only 750 μm.